It is a common practice in the oil and gas industry to use downhole tools in completed and producing oil and gas wells. These downhole tools vary in purpose, but are generally used for well maintenance, flow control, or safety. To illustrate the operating mechanism of a typical downhole tool, the operating system of a subsurface safety valve is described below.
A subsurface safety valve is generally located deep in a producing well and is often included in a production tubing string. Well fluid flows through the production tubing string and the subsurface safety valve to the well surface, where the well fluids are diverted through valves into pipes for transport or storage. The subsurface safety valve acts as a downhole flow control device to block well fluid during emergency conditions. The subsurface safety valve is typically designed to allow well fluid to flow through the production tubing string to the surface when positive control is exerted over the subsurface safety valve from the well surface. Typical surface controlled subsurface safety valves are disclosed in U.S. Pat. No. 4,527,631, entitled Subsurface Safety Valve and U.S. Pat. No. 4,325,431, entitled Flow Controlling Apparatus, each of which is incorporated into this application by reference.
The positive control over the subsurface safety valve is generally in the form of hydraulic pressure applied at the well surface, which is communicated through a conduit to a hydraulic piston contained within the subsurface safety valve. The hydraulic pressure from the well surface opens a valve member contained within the subsurface safety valve and allows well fluid to pass through the production tubing string to the surface of the well. When positive control, or hydraulic pressure, is removed, the subsurface safety valve will close and stop the flow of well fluid through the production tubing string. During an emergency, the hydraulic pressure is removed either by an actuator at the well surface or by damage to the hydraulic conduit, thereby stopping the flow of well fluid to the well surface.
Many subsurface safety valves are constructed using a flapper type valve member to open or close the well flow. A flapper type valve is generally opened by a sleeve which is directly connected to a hydraulic piston within the subsurface safety valve. The hydraulic piston is activated by hydraulic pressure from the well surface which extends the sleeve to a second position to open the flapper type valve and hold it open, allowing unrestricted flow of the well fluid to the surface. A spring or other biasing system is often used to bias the sleeve in a non-extended or first position, thereby allowing well fluid to close the flapper type valve member and block well fluid flow to the well surface when hydraulic pressure is removed from the hydraulic piston.